What is Zapping PRAM in Mac OS X?
Sometimes we may heard Macintosh computer technicians say a computer needs its PRAM zapped. For most of us, this sounds very odd, and maybe even a little obscene. But don't worry; it's a standard part of the Mac troubleshooting routine.
PRAM stands for "parameter random access memory." A document in Apple's knowledge base reads:
"PRAM stores certain system and device settings in a location that Mac OS X can access quickly. Exactly which settings are stored in the computer's PRAM varies depending on the type of computer as well as the types of devices and drives connected to the computer. Parameter RAM is a small area of non-volatile RAM (NVRAM)."
Because PRAM is "Non-volatile RAM," it will store data even when the computer is turned off. It will also typically store data when the computer automatically shuts off due to severe battery drain.
According to Apple, some of the data stored in PRAM includes:
- Display and video settings such as refresh rate, screen resolution, number of colors
- Time zone setting
- Startup volume choice
- Speaker volume
- Recent kernel panic information, if any exist
- DVD region setting
Many issues can be resolved by "zapping the PRAM." This includes issues such as Macs that are flashing a question mark when powered on, to missing pictures on the iMac G5's display, to computers that simply won't power up at all. PRAM can be corrupted by faulty (typically third party) software, power surges, and electromagnetic interference.
In most Macs you can "zap the pram" by powering the computer on, then immediately holding down the Command, Option, P, and R keys simultaneously. The Command key is the one with the cloverleaf or Apple on it.
According to Apple, OS X does not store network settings in PRAM. If you experience a network issue, resetting PRAM will not help.
If PRAM is reset, you may need to verify your time zone, startup volume, and volume settings using System Preferences. Certain firmware updates may reset PRAM as a normal part of their installation process."
Note that zapping PRAM is different than resetting the Power Management Unit (PMU,) which you might have to do on certain Apple laptops.
Apple says "A PMU reset should not be necessary except as a last resort in cases where a hardware failure of the power management system is suspected. Performing a PMU reset returns the iBook and PowerBook hardware, including NVRAM, to default settings and forces the computer to shut down."